Although low sulfur, sub-bituminous coal is readily available it may have such a high moisture content and low heating value that it is of little use as a fuel. Thermal drying to reduce the moisture content of the coal can upgrade its heating value to a point where the dried coal can compete favorably with many bituminous coals. With a low sulfur content such coal can meet clean air requirements for many power plants and make a major contribution to reducing sulfur dioxide emissions and acid rain. The drying required with such low rank coals is a deep drying process to remove both surface water and large quantities of interstitial water present. The handling, storage and transportation of such deep dried coal can present technical problems resulting from the friability and dustiness of the coal, as well as its tendencies to readsorb moisture and react with oxygen from the air. Spontaneous combustion can result from heats of moisture readsorption and oxidation. Removing moisture inherent in the coal structure can also reduce the strength of the coal particle by cracking or fissuring, causing friability and dusting. The number of active surface sites exposed within the coal particles can also thus be increased, thereby increasing undesired moisture adsorption and oxidation.
It has been previously proposed to spray dried coal with hydrocarbons, to countereffect the effects of too rapid drying and reaction with atmospheric moisture and oxygen. For example, U.S. Pat. No. 4,201,657 discloses a composition for treating coal and lignite wherein the material is treated with a composition made up of a blend of aromatic hydrocarbon oil and asphalt. Similarly, U.S. Pat. No. 4,265,637 discloses treating lignitic type coal after it has been dried with a hydrocarbon liquid having a minimum flash point of 65.degree. C. such as No. 6 fuel oil, residual oil, heavy distillates and other low value refinery streams. U.S. Pat. No. 4,396,394 discloses the use of a deactivating fluid to treat a dried, partially oxidized coal. U.S. Pat. No. 4,402,707 discloses the treatment of dried coal with a virgin vacuum reduced crude oil.